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The (n,ϵ)-pseudospectrum of an element of a Banach algebra
K. Dhara,
Published in Academic Press Inc.
2018
Volume: 464
   
Issue: 1
Pages: 939 - 954
Abstract
Let A be a complex unital Banach algebra, a∈A, n∈Z+ and ϵ>0. The (n,ϵ)-pseudospectrum Λn,ϵ(a) of a is defined as Λn,ϵ(a):=σ(a)∪{λ∉σ(a):‖(λ−a)−2n‖1/2n≥[Formula presented]}. Here σ(a) denotes the spectrum of a. The usual pseudospectrum Λϵ(a) of a is a special case of this, namely Λ0,ϵ(a). It is proved that (n,ϵ)-pseudospectrum approximates the closed ϵ-neighbourhood of spectrum for large n. Further, it has been shown that (n,ϵ)-pseudospectrum has no isolated points, has a finite number of connected components and each component contains an element from σ(a). Some examples are given to illustrate these results. © 2018 Elsevier Inc.
About the journal
JournalJournal of Mathematical Analysis and Applications
PublisherAcademic Press Inc.
ISSN0022247X